Flow cytometry is one of the most widely used and indispensable techniques for analysis of cell phenotypes and for isolation of specific cell populations. The Flow Cytometry Core resources supported by the P01 provide the Program's investigators with increased access in a cost effective way to instruments for analytical flow cytometry, which in some cases must be maintained in appropriate biosafety containment conditions. The core has three specific aims: 1) Provide and maintain the necessary research infrastructure needed for flow cytometry studies, with appropriate biosafety containment safeguards; 2) Provide technical assistance in the performance and interpretation of established techniques involving flow cytometry; 3) Introduce new and update existing technologies to maintain cutting edge capabilities. The core facility supported by the P01 has full responsibility for maintaining one analytical flow cytometer, and contributes to the maintenance and operation of one high speed cell sorter. Access through this core to FACS analysis and sorting techniques under Biosafety Level (BSL)-2 and BSL-3 conditions has played an essential role in research funded by this P01 grant during the past 5 years, and will continue to do so into the next funding period. The high cost ofthe instrumentation and the need for skilled personnel makes the establishment of a core facility for flow cytometry the only practical approach to management of these resources. In addition, this core facility provides the only possibility for the P01 investigators to do flow cytometry in a BSL-3 environment within the institution, and thus is a crucial facility for some of the studies that are ongoing or planned within the context of the program. The core also provides an environment in which biosafety containment procedures can be strictly overseen and enforced, thus ensuring the safety of personnel using flow cytometry techniques in their work with potentially dangerous infectious agents. The analytical capabilities of the core allow the program investigators to identify and characterize cell populations of interest in a wide range of studies related to mycobacterial pathogenesis and the host immune response.